1. Field
The present invention relates to an air-fuel ratio estimating/detecting device, and more particularly, to an air-fuel ratio estimating/detecting device that can detect a wide-range of air-fuel ratio by estimation without using a so-called wide-range air-fuel ratio sensor.
2. Description of the Related Art
There has been known a technology of indirectly detecting an air-fuel ratio (hereafter, also referred to as “A/F”) by detecting the concentration of oxygen in the exhaust gas of an engine and performing combustion control of the engine, including ignition control or fuel injection control, on the basis of the detection result. Further, as an oxygen concentration sensor that is a detecting element detecting the concentration of oxygen in the exhaust gas, a so-called λ-sensor of which the electromotive force, that is, the detection output is rapidly changed (in a stepwise fashion) at the interfaces of the oxygen concentration corresponding to a theoretical air-fuel ratio (air excess ratio=1) is widely used, due to the simplicity. According to the λ-sensor, it is possible to easily determine whether the air-fuel ratio is larger or smaller than the theoretical air-fuel ratio.
However, the λ-sensor, which detects the oxygen concentration only from the difference of the air-fuel ratio from the theoretical air-fuel ratio, cannot accurately detect the air-fuel ratio in the area departing from the theoretical air-fuel ratio. Therefore, the λ-sensor cannot be used control setting the air-fuel ratio into an optional value including the rich side and the lean side regions, other than the theoretical air-fuel ratio. Meanwhile, the wide-range air-fuel ratio sensor that can detect air-fuel ratio within a wide-range is expensive, because the structure is complicated.
Therefore, an air-fuel ratio estimating/detecting device that estimates an air-fuel ratio on the basis of the crank angular speed has been proposed, without using an oxygen concentration sensor, as disclosed in Patent Literature 1 (JP-A-2001-27061).
According to the air-fuel ratio estimating/detecting device described in Patent Literature 1, it is possible to estimate the air-fuel ratio without using an oxygen concentration sensor, and appropriately perform ignition control or fuel injection control on the basis of the estimated value. However, only the estimation of the air-fuel ratio based on the crank angular speed may be insufficient and means for estimating an air-fuel ratio with high accuracy is required.